1. Field of the Invention
The present invention relates to variable displacement compressors that are used in vehicle air conditioners. More particularly, the present invention relates to a variable displacement compressor that changes its discharge displacement by adjusting the inclination of a cam plate.
2. Description of the Related Art
Variable displacement compressors typically have a cam plate that is tiltably supported on a rotary shaft. The inclination of the cam plate is controlled based on the difference between the pressure in a crank chamber and the pressure in cylinder bores. The stroke of each piston is varied by the inclination of the cam plate. The crank chamber is communicated with a discharge chamber by a supply passage. A displacement control valve is located midway in the supply passage. The displacement control valve controls the amount of refrigerant gas introduced into the crank chamber from the discharge chamber.
Opening the control valve allows refrigerant gas to flow into the crank chamber from the discharge chamber via the supply passage. This increases the pressure in the crank chamber, thereby increasing the difference between the pressure in the crank chamber and the pressure in the cylinder bores. This minimizes the inclination of the cam plate. Accordingly, the displacement of the compressor becomes minimized. On the other hand, closing the control valve stops the flow of refrigerant gas from the discharge chamber to the crank chamber. This reduces the pressure in the crank chamber, thereby decreasing the difference between the pressure in the crank chamber and the pressure in the cylinder bores. This maximizes the inclination of the cam plate. Accordingly, the displacement of the compressor becomes maximized.
During operation, sliding parts in compressors often abrade one another and thus generate metal powder. High temperature and pressure accompanying gas compression often carbonize lubricant, thereby generating carbide in the compressor. In many variable displacement compressors, a control valve located midway in a supply passage includes an aperture that opens and closes for selectively allowing and stopping the flow of refrigerant gas. The cross-sectional area of the aperture is extremely small. Refrigerant gas passing through the opening of the valve contains lubricant oil. Therefore, if foreign matter such as metal powder and carbide is mixed with the lubricant oil, the foreign matter may choke the aperture. The choked aperture disables the displacement control by the control valve.
Variable displacement compressors often have a rotary shaft directly connected to an external drive source such as an engine without a clutch located in-between. In this clutchless system, when the control valve is opened such that the compressor operates at the minimum displacement, refrigerant gas flow from an external refrigerant circuit into the compressor is stopped. However, refrigerant gas continues to circulate within the compressor. Lubricant oil contained in the refrigerant gas also circulates within the compressor and is not discharged to the external refrigerant circuit. Therefore, if foreign matter is mixed with the lubricant oil, the circulation of the lubricant oil abrades sliding parts in the compressor. Further, if foreign matter chokes the aperture of the control valve, the compressor continuously operates at a large displacement even when cooling is not needed. That is, the air conditioner becomes uncontrollable.
In order to solve the above drawback, some variable displacement compressors are equipped with a filter at the inlet of the control valve. The filter filters out foreign matter such as metal powder and carbide mixed with lubricant oil in refrigerant gas when the gas flows from the supply passage to the control valve.
If the filter is choked with foreign matter, the flow of refrigerant gas in the supply passage will be restricted, and the displacement of the compressor cannot be accurately controlled. Therefore, foreign matter accumulated on the filter needs to be removed. However, since the filter is located at the inlet of the control valve located midway in the supply passage, the control valve needs to be detached from the compressor in order to remove foreign matter from the filter. This makes filter cleaning burdensome. Also, since the supply passage is narrowly formed, foreign matter is likely to accumulate on the filter.